Various processes for recovering paper fibers from scrap paper such as paperboard food cartons and packages, office waste paper, magazines and newspapers have been proposed. Typically, these recycle processes involve treating the waste paper in a hydrapulping machine wherein the fibers are separated by the agitation of the water, and the use of caustic soda or similar reagents break down the integrity of the waste paper. This results in a stream of separated paper fibers which may be passed through various screening devices to remove contaminants, and the resulting slurry containing the paper fibers may pass through a de-inking process. After appropriate treatment, which may include bleaching, the slurry of paper fibers passes through a de-watering stage, so that the recovered paper fibers are collected in an essentially dry state and may be packed in bales for subsequent use in making paper. An example of a recycle process that would be suitable for obtaining dry paper fibers is disclosed in U.S. Pat. No. 5,390,860, "Method and Apparatus for Separating Paper Fiber and Plastics from Mixed Waste Materials", which is incorporated herein by reference.
If recovered paper fibers are to be used for making packages or containers for food products, government regulations require that they meet the same standards as for virgin paper fibers, e.g., the recommendation by the International Dairy Federation (IDF) of a maximum bacterial load of 250 colony-forming units per gram ("cfu/g").
In a typical paper plant, virgin pulp is digested and bleached in a slurry that is supplied directly to the paper-making machine. Recycled paper fibers are often processed in local recycle plants, dried, stored in bales, and subsequently transported to paper-making plants for further processing into cartons and containers. While the bales of fibers are being stored, microorganisms that are present may multiply and cause contamination of the final paper product. It has been proposed to sterilize recycled fibers by heating the fibers to a pasteurizing temperature. However, this sterilization process requires steam-heating equipment and vessels, and thus can be very expensive. Addition of bactericides to a dilute pulp slurry has also been proposed to control bacterial load in food packaging, but also requires substantial input of energy and chemicals to achieve the desired results. Moreover, use of bactericides may leave undesirable residues in the pulp. The efficiency and economy of the recycle process is important because of the substantial costs incurred in obtaining the recycled fibers. For recycling to succeed as an economically feasible process, these costs must be as low as possible while achieving the desired sanitary standards.
Among the microorganisms most difficult to kill are spore-forming bacteria. Spore-formers can survive the drying temperatures which are normally employed in a paper-making machine. If these spore-formers are present in the paper fibers that are supplied to the paper-making machine, the resulting paper and paperboard are likely to have substantially more than the 250 limit of colony-forming units per gram ("cfu/g").